The present invention generally relates to devices that measure the location of a rotatable member. More particularly, the present invention provides a device and method, capable of measuring the optical rotary and linear position of items, and that is reliable, accurate, immune to EMI and capable of operating in harsh environments.
Many scientific, industrial, military and aerospace applications require precise and accurate knowledge of the angular orientation of a shaft or other rotating object or the linear position of a reciprocating object. This is a task often complicated by high temperatures, magnetic interference and vibration. A variety of rotation sensors are currently available in the art for sensing or measuring relative rotation of a rotatable member. These devices generally record very small angular changes and keep track of accumulated change relative to some reference angle. In these encoders the angular information generally is lost if this reference angle becomes corrupted. This may occur for a number of reasons such as power interruption or magnetic interference. Also, some of these encoders achieve additional sensitivity by means of gear trains which are subject to hysteresis which limit accuracy and make angular determination indirect. Another problem with devices that rely on low level electrical signals to determine the distance and/or angular displacement between two parts is inability to deliver accurate reading at elevated temperature such as between 500° F. and 1000° F.
Another type of rotation sensor is an optical encoder. In such a rotation sensor, a matched light source and a photodetector are provided on the opposite sides of a disk. The disk is attached to a shaft which is in turn mechanically coupled to the device whose rotation is to be measured. The disk is provided with a plurality of apertures which vary in radial position as the disk rotates, enabling light to pass through only at certain radii corresponding to its degree of rotation. Optical encoders are commonly available to provide absolute or incremental angle measurement. Although optical encoder rotation sensors function satisfactorily for certain applications, their use in many applications is limited because of size and weight limitations. They are also sensitive to electromagnetic interference (EMI).
U.S. Pat. No. 5,329,121 issued to Leviton describes an optical encoder. The inventor discloses a device for positioning encoding of a rotating shaft in which a polygonal mirror having a number of facets is mounted to the shaft and a light beam is directed towards the facets. The facets of the polygonal mirror reflect the light beam such that a light spot is created on a linear array detector. An analog-to-digital converter is connected to the linear array detector for reading the position of the spot on the spots on the linear array detector. A microprocessor with memory is connected to the analog-to-digital converter to hold and manipulate the data provided by the analog-to-digital converter on the position of the spot and to compute the position of the shaft based upon the data from the analog-to-digital converter. This device has significant disadvantages in high temperature operations such as jet engines and gas turbines in that the measuring device may be dirtied. For apparatus that rely on intensity or analog measurements this can cause great difficulty. Dirt and debris can significantly interfere with the measurements and the equipment. This can lead to misinformation. Further, contamination at the sensing end and connectors must be cleaned and/or replaced in the field. This is time consuming, costly, and overly difficult.
Rotation sensors utilizing electrical resolver technology continue to be used in some aircraft applications. Electrical resolvers rely on mutual induction of electrical coils to convey the rotational position. Rotation sensors using this technology are generally quite massive, are usually less accurate and more expensive than sensors using optical encoders, and are also sensitive to EMI.
Resistive potentiometers are also used in some rotation sensors. The principal limitation of this type of rotation sensor is the limited number of cycles per lifetime, particularly with small sensors. Further, the output of the potentiometer, in most applications, requires shielding. Although normally smaller in size than an optical encoder sensors, they are often less desirable for small volume, critical applications because of unreliability and reduced precision. A further major drawback to potentiometer based sensors, like the others described above, is that the signal is conveyed via a wire; therefore, they are not immune to electromagnetic interference (EMI).
Hall effect rotation sensors involving the rotation or movement of a magnet past a detector are also available. However, they likewise suffer from the limitation of not being immune to EMI.
As can be seen, there is a need for a device and method capable of measuring the optical rotary and linear position of items that is reliable, accurate, immune to EMI and capable of operating in harsh environments.